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Alternating between periods of eating and fasting is gaining in popularity among dieters and generating criticism in nutritional circles.

Intermittent fasting, sometimes known as the 5:2 diet, asks people to eat very little or nothing at times, such as eating normally for five days a week and fasting for the other two.

Intermittent fasting is designed to set up the body to burn fat, says Brad Pilon. (CBC)

Brad Pilon designed one of the first intermittent fasts that became popular after he published a guide, Eat Stop Eat. Pilon said the diet allows followers to eat the foods they crave most of the time and still lose weight.

"In the fasted state your body's set up to burn the calories you stored while eating," said Pilon. "So it's set up specifically for the act of burning body fat."

Cutting down on weekly calorie intake is generally recommended. And there's research underway into the hypothesis that restricting calories could extend a healthy lifespan.

Intermittent fasting is thought to be easier to stick to then continuously watching what you eat. (Brian Snyder/Reuters)

Critics of intermittent fasting say that besides burning unwanted fat, the body will also burn its building blocks.

"So when those energy stores start to drop the body looks for other sources and it goes to the muscles and burns muscle," said Margaret De Melo, a registered dietician at Toronto Western Hospital.

History is full of similar examples of people dieting, falling off and dealing with the effects that yo-yo dieting creates, De Melo said.